The intake manifold temp sensor triggers the fans to run at a noticeably slower speed only when the intake manifold is quite hot and only when the ignition is off. A fault in that sensor or that circuit should not cause the fans to run at full speed at any time, and should not cause the fans to run when the ignition is on.
I believe that a faulty AC refrigerant switch could cause the fans to run at full speed while the ignition is on. Be sure you know which sensor on the AC receiver/dryer is the temp switch, and which is the pressure switch. It also might be possible for the radiator switch to cause this. Disconnecting the switches one at a time should show if either is the source of the problem.
Wally
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This is the power flow for a 1988 model

 

Letís think sensors:
- The AC Freon pressure sensor at the AC dryer. It is temp resistor
o Pressure above 4 bar: sensor reads about 40 ohms and fans should run moderate speed
o Pressure above 15 bar: sensor reads 120 ohms and fans activate to full speed
 

- The coolant temp sensor in the front of the radiator. This is a temp resistor
o Coolant hot: about 1100 ohms value for full speed fan activation (95C)
o Coolant moderate: below 3800 ohms, fans run at moderate (87C)
o Coolant cold: over 3800 ohms, fans stop as the system seems to have cooled down enough (83C)
 

- The AC switch in the dashboard. This is the pushbutton that switches the AC on.
o Switch pressed: 12v from the Fresh air relay is sent to the controller
o Switch off: 0v is sent to the controller
 

- The intake temp switch
o Temp of the intake above 90C: switch engages and connects gnd to the controller
o Temp of the intake below 90C: switch disengages and disconnects from the controller
 

- Transmission oil temp switch
o Switch engages at 110C and connects gnd to the controller
o Switch disengages below 110C and disconnects from the controller
 

- Door contact engine hood (1987 up to the 1991 model)
o Switch engaged (hood closed) enables the fans running
o Switch disengaged (hood open) disables fans from running (safety measure)

Theo

1992 928 GTS midnight blue

http://928gts.jenniskens.eu